Electrical power is distributed from central generating plants to homes, offices, and factories as three-phase alternating current. Concerns for the environment have motivated engineering research to improve power generation efficiency. In addition, there have always been efforts directed at improving power system efficiencies by reducing line losses. Since the power lost in the phase conductors is given by the square of current times the resistance, it is obvious that either increasing conductor diameter or the number of conductors (known as using a conductor "bundle") will reduce the "I squared R" losses. However, making these changes by themselves for three phase powerline does not change the fringing magnetic field under and near the powerline. Thus, eddy current losses in nearby water pipes, losses from inductive coupling to fences, etc., will not be changed. Furthermore, increasing wire diameter or adding paralleled conductors on the same crossarms will significantly increase the fringing electric field.
In recent years there has been an increasing public concern about possible biological effects of the low frequency electric and especially magnetic fields associated with the distribution and use of electrical power. If an engineering change is made to improve efficiency which, as in the example discussed above, will carry the disadvantage of increasing fringing electric or magnetic fields, it will meet with strong political opposition. Efficiency improvements must be coupled with fringing field reduction.
In discussing public health issues related to power lines, it is necessary to consider two classes of power lines:
1) Urban distribution lines running from a substation to distribution transformers located near the point of use, and commonly operated at 10 to 50 kV. PA1 2) Urban and rural transmission lines that supply power from generating stations to substations and that are commonly operated at voltages above 69 kV.
Although much of the most recent public outcry has been directed at the second category of power transmission lines listed above, the 10-50 kV distribution lines may be more significant because of the vastly greater number of people exposed. Distribution lines in the 10-50 kV category, if mounted on poles, can give rise to electrical fields of as much as 20 V/m, and magnetic fields as high as 1.3 micro-tesla when measured on the ground below the lines. Maintenance workers, such as tree trimmers who work near the 10-50 kV distribution powerlines, receive a significantly higher exposure to both electric and magnetic fields.
The issue of fringing fields from 10-50 kV lines has a well known solution that is not always economically feasible--underground utilities. Both theory and measurements show that placing all three phase conductors in a common conduit provides nearly perfect magnetic and electrostatic shielding. Underground utilities are also recommended by their improved reliability, appearance, and safety from accidental electrocution. These factors have led to their widespread use in new construction. A major question in the controversy over possible health risks from fringing fields is whether to rebuild existing distribution lines and to literally bury the supposed problem. A method of reducing or eliminating fringing fields while using existing pole-supported open wires could provide an economically attractive alternative method of resolving the political problem.
The relative magnitude and rate of fall-off of both the fringing electric and magnetic fields is directly proportional to conductor spacing and, for distances beyond about 10 times the conductor spacing, inversely proportional to distance squared. For example, one can consider the significant difference of the fringing magnetic field measured near two types of 120/240 service drops to residences. In installations made before about 1950, all three wires running from the utility pole to a house were supported on individual insulators. Conductor separation was of the order of 30 cm. As insulating material became more weather resistant, this construction was replaced by a cable consisting of a support wire and the two "hot" wires, which are wrapped around the support wire in a spiral fashion. In this newer design, conductor separation is reduced to about 3 cm. This reduction of conductor-conductor spacing reduces fringing fields measured on the ground under the service drop to about 1/10 of the value found for the old wiring technique and increases the rate of fall-off away from the service drop.
The prior art three-phase distribution powerlines often have banks of capacitors distributed along the circuit. The purpose of these capacitors is to draw a leading current to offset the lagging current of inductive loads, such as refrigeration motors. The leading current offsets the lagging current to bring the power factor closer to unity and minimize the total current in the distribution powerline, thus minimizing powerline I squared R loss.
The foregoing discussion is directed toward powerlines made of several parallel conductors. Another widely used transmission line design is "coaxial", which is widely used when electrostatic shielding is important. The adaptation of coaxial lines to the fringing field-free transmission of three-phase electric power has been discussed in my co-pending application (U.S. Ser. No. 07/578,215), which is incorporated herein by reference.
Coaxial transmission lines have a higher capacitance per unit length than do parallel lines, and will hence have a higher leading power factor. In a 10-50 kV distribution line this may be an advantage, as both fluorescent lights and induction motors operate with a lagging power factor that the coaxial line may offset. Consequently, the added capacitance of the coaxial transmission line reduces power loss by correcting lagging power factors.
It is instructive to consider the fringing magnetic fields associated with both the parallel wire and coaxial transmission lines. For the two parallel wire case one can show from electromagnetic theory that, for all locations that are further away from the transmission line than 10 times the inter-conductor spacing, the magnetic intensity is directly proportional to line spacing and inversely proportional to the square of distance. That is, near the conductors the electric and magnetic fields are intense, but once one moves away more than 10 times the conductor spacing, the fields become rapidly weaker. Magnetic intensity varies inversely with the square of distance. Thus, for prior art three phase power transmission lines, the general rule is that reduction of fringing fields accompanies close conductor spacing. This accounts for the negligible magnetic fields on the surface of the ground above buried distribution powerlines. Of course, other factors enter into the engineering choice of conductor spacing. Thus, reducing fringing fields by reducing conductor spacing has some very practical limitations such as arcing between phase lines.
For the case of the coaxial line, one finds complete electrostatic shielding of the inner conductor by an outer conductor, and transmission line theory shows that if the return current flows in the outer conductor, there is no magnetic field outside the transmission line. This holds even for the outer conductor material being copper or other non-ferromagnetic material. For three phase power lines, the use of a separate coaxial line for each phase would not eliminate fringing magnetic fields, because one would generally find that the return current flowing in the outer conductor was not equal in magnitude to the forward current. In a co-pending patent application, OVERHEAD THREE-PHASE POWER LINE ELIMINATING FRINGING ELECTRIC AND MAGNETIC FIELDS, Ser. No. 07/578,215, Sept. 6, 1990, the present inventor taught methods of insuring an equality between the forward and reverse currents in the two conductors. The teachings of Ser. No. 07/578,215 have a further significant disadvantage, in that the return current flowing in the outer conductor of the coaxial cable consumes power which would not normally be consumed in three-phase power transmission.